diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -454,8 +454,10 @@
                                          SourceLocation::UIntTy TotalSize) {
   assert(ExternalSLocEntries && "Don't have an external sloc source");
   // Make sure we're not about to run out of source locations.
-  if (CurrentLoadedOffset - TotalSize < NextLocalOffset)
+  if (CurrentLoadedOffset < TotalSize ||
+      CurrentLoadedOffset - TotalSize < NextLocalOffset) {
     return std::make_pair(0, 0);
+  }
   LoadedSLocEntryTable.resize(LoadedSLocEntryTable.size() + NumSLocEntries);
   SLocEntryLoaded.resize(LoadedSLocEntryTable.size());
   CurrentLoadedOffset -= TotalSize;